The Great Conjunction of Saturn and Jupiter is occurring tonight, but what does that actually mean? Starting with the basics, think of the solar system as a race track. The planets closer to the sun travel faster than those that are further away. Jupiter travels about 2.5 times faster than Saturn and passes it about every 20 years. That's called a conjunction.
But what makes this year's conjunction a "great" one? Again, thinking of it as a race track, the track isn't flat all the way around. Near the edges, it tilts upwards. The same applies to the orbits of the planets. Saturn's orbit will be less tilted, allowing the planets to appear closer together.
You can view the bright planets in the southwest sky about an hour after sunset for the next 2 nights. This will be the closest conjunction in 400 years, so try to see it if you can!